[Morphological study of muscle fibers stained red by modified Gomori trichrome staining with special reference to smooth red fibers].

The modified Gomori trichrome stain of muscles can demonstrate ragged red fibers which are irregular in outline and display a thick and irregular red subsarcolemmal layer and intermyofibrillar red deposits. Typical ragged red fibers are often encountered in mitochondrial myopathy. On the other hand, we have noticed fibers outlined by a thin red subsarcolemmal layer. These fibers are smooth in outline. The sarcoplasm shows normal intermyofibrillar network. We defined these fibers as "smooth red fibers". To investigate the pathological significance of the smooth red fibers, we studied morphological differences between the smooth red fibers and ragged red fibers by light and electron microscopy and evaluated the occurrence and characteristics of the both abnormal muscle fibers in several neuromuscular diseases. Muscle specimens from 738 patients who were seen or consulted at the Department of Neurology, Hokkaido University, from January 1980 to October 1994 were examined. The smooth red fibers were classified into two types, type I and type II. Type I smooth red fibers were hypertrophied and showed a thin smooth red margin. Electron microscopy of the type I smooth red fibers showed no mitochondrial abnormality, being different from ragged red fibers which have abnormal mitochondria. Type I smooth red fibers were observed in chronic denervation process; they were specially frequent in Kugelberg-Welander syndrome. Hypertrophy of type I smooth red fibers were considered to be a compensative reaction in chronic denervation. Type II smooth red fibers were observed with or without ragged red fibers in mitochondrial myopathy. Type II smooth red fibers showed a thin smooth red margin, spreading red deposits from the margin into sarcoplasm. The fibers showed mitochondrial abnormality in electron microscopy. It could be posturated that type II smooth red fibers were transformed into ragged red fibers. The findings suggest 1) type I and type II smooth red fibers are different in origin, 2) type II smooth red fibers change to ragged red fibers, 3) type I and type II smooth red fibers are important markers of chronic denervation and mitochondrial myopathy, respectively. These two types of smooth red fibers are worthy of attention in muscle pathological observations.